The intracellular concentrations of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) were determined in adult male Holtzman rat small intestinal tumors induced by subcutaneous administration of 1,2-dimethylhydrazine (DMH) or X-irradiation of only the hypoxic ileum and jejunum. The levels of cAMP and cGMP in the cancer cells from DMH-treated animals were 50% and 200%, respectively, of the values measured for intestinal tissue. The amounts of cGMP in the lesion of the X-irradiation induced rat small bowel adenocarcinoma determined in the present investigation and of cAMP measured previously were observed to be only 50% of the value for unirradiated intestine. It has thus been shown that in the DMH-induced colon and small intestinal tumors, the X-irradiation induced rat small bowel adenocarcinoma and the human colon adenocarcinoma, the cAMP concentrations are consistently about 50% of the levels measured in comparable normal tissues. These findings of diminished intracellular cAMP concentrations imply a serious cellular defect mechanism occurring in intestinal cancer. However, there appears to be no similar pattern of change for the intracellular concentrations of cGMP, which suggests that different biochemical pathways exist in these malignancies.